Mitchell succeeds with light sand fracing in Barnett shale

Sam FletcherOGJ Online

HOUSTON�Using the new light sand fracturing technology to enhance production at lower cost, Mitchell Energy & Development Corp. has accelerated development of the Newark East Barnett shale field in North Texas, doubling gas production from that field to 172 MMcfd in just over a year.

Mitchell Energy has long made use of hydraulic fracturing to improve production from tight sand developments or aging wells, including some of the most massive frac jobs on record in the early 1980s.

"Over the last 2 years, we've gotten away from those massive fractures that used to use 1.2 million lb of sand and 600,000 gal of 30-50 lb gels," said Mark Whitley, vice-president and general manager of Mitchell's prolific North Texas operations.

Now, he said, the company can get better results from the light sand fracturing techniques�also known as "waterfracs"�that others have proven so successful in East Texas, using large volumes of water and a relatively minimal 100,000 lb of sand. Moreover, the new light sand technology has reduced total well costs by 20%, allowing the company to expand the field limits and complete the previous uneconomic Upper Barnett interval. That makes the development program lucrative even at market prices of $2/Mcf, said Whitley.

Reserves growthThe Newark East field, some 40 miles north of Fort Worth, produces from Mississippian shale located at 7,000-8,000 ft that averages 550 ft of pay. The formation is divided by a layer of impervious rock into upper and lower units that both contain gas. But the old hydraulic fracturing methods were too expensive and time consuming to use in developing the Upper Barnett formation, Whitley said.

With light sand fracture technology, Mitchell Energy has added Upper Barnett production to about 400 existing wells, compared with only 40 that were producing from the formation less than a year ago as a result of light sand fracing (OGJ, Sept. 27, 1999, p. 89). All of the new wells drilled since late 1998 have Upper Barnett production.

The program has increased per-well recoverable reserves by 250 MMcf, or some 33%, says Mitchell.

The US Geological Survey in 1998 estimated the Barnett's recoverable reserves at 10 tcf of gas, the equivalent of a 1.67 billion bbl field. It accounts for about 45% of Mitchell's total production, officials said.

"Over the last few years, our estimated recovery efficiency in the Barnett has more than doubled, but we are still only recovering less than 10% of the total gas in place," said George P. Mitchell, chairman and CEO of Mitchell Energy, based in The Woodlands north of Houston.

"Light sand fracture technology has helped us tremendously in our efforts, but we continue to look for new ways to increase reserves," he said.

Mitchell Energy is accelerating its drilling program, which has added 153 wells to the field since late 1998. The company currently has six rigs working in that field and plans to drill at least 140 new wells annually. Officials are already looking at contracting more rigs for that field next year.

RefracturingIn its effort to boost its per-well drainage area and increase total recovery from the Barnett, Mitchell Energy is re-fracturing mature wells.

Among the wells that workers reentered was the field's oldest, the C.W. Slay discovery, which had been temporarily abandoned as uneconomical in September 1998. After refracturing, that well began producing more than 1.1 MMcfd of gas, up from a previous level of 250 Mcfd, said Whitley.

"We knew we had found a crucial key to unlocking the potential of this tight gas reservoir when the results of our light sand fracture completion technology came pouring in. But now we're also able to use this technology to significantly extend the life of a well and increase the amount of reserves recovered per well," said Mitchell.

"The benefits of this combination are magnified when you consider that we likely have another 2,000-5,000 undrilled locations even after working the Barnett play for nearly 20 years," he said. The company has more than 200,000 acres under lease in that field.

To fulfill that potential of 5,000 undrilled locations, the company has started three pilot programs to evaluate the impact of reduced well spacing. Initial results show that a reduced spacing of about 27 acres, vs. present spacing of 55 acres, allows a well to maintain as good production, with no adverse effects on offset wells, officials said.

In fact, production rates at some nearby wells have actually increased "several-fold" as a result of refracturing, they said.

A decision on whether to move aggressively on a field-wide spacing reduction will be made early next year.

Meanwhile, a program to expand Mitchell Energy's Bridgeport gas processing plant by 50% is on target for completion in early December. That additional capacity is required to process the growing production from the Barnett.

Under that program, the plant's inlet capacity will be increased by 100 MMcfd, adding another 8,000 b/d to its NGL extraction capabilities.